1. Field
The following description relates to a technology of sensing bio-signal information and signal processing by radiating X-rays to a subject to obtain an internal image thereof.
2. Description of the Related Art
Since its discovery by Roentgen in 1895, X-rays have been a useful tool for looking into internal organs of the human body without dissection. Such X-rays have been widely used in many fields, such as orthopedics and cardiothoracic surgery, and the development of a computed tomography (CT) in the early 1970s ushered in a new era of three-dimensional images of internal organs of the human body obtained by tomography imaging.
An X-ray imaging method basically uses a mechanism in which x-rays are absorbed into the tissues of the human body. Depending on thickness of body tissues, and difference in X-ray absorption by tissues, signal strength received on a two-dimensional sensor (X-ray film or digital sensor) is different at every location, and such difference is expressed in gray levels of images.
When X-rays are absorbed into the human body, atoms are temporarily ionized, as a result of which body tissues or DNA binding may be damaged. Therefore, when the human body is exposed to intense, or a huge amount of X-rays, side effects including organ damage may occur. Accordingly, the department of radiology limits an X-ray dose exposed to the human body, and recommends a minimum use thereof.
In X-ray imaging of hands or feet, which are thin body parts, a small dose of X-rays is used. However, for thick body parts, such as pelvis or chest, a large dose of X-rays is required so that sufficient X-ray signals are sensed on a sensor side to get high quality X-ray images. However, an X-ray dose of CT imaging is in principle hundreds of times higher than that of a simple X-ray imaging, since CT imaging is conducted in such a manner that hundreds of X-ray images are captured while rotating 180 to 360 degrees around the human body to be reconstructed afterwards as tomographic images of internal human body. For this reason, even with a one-time CT imaging in the early 2000s, a person was exposed to an X-ray dose nearly equivalent to an amount a normal person is allowed to be exposed to in one year. Therefore, CT manufacturers have competed and made every effort to reduce an x-ray dose, and equipment is recently being released that reduce x-ray dose to a tenth of the existing equipment, with almost no image degradation.
The effort to reduce x-ray dose in CT imaging should be approached by considering the aspects of an X-ray source, a detector, and an algorithm for image reconstruction.
In an aspect of X-ray source, a method for emitting X-rays with a narrow energy band is being developed. X-rays are generated in such a manner that electrons are accelerated to collide with a metal, which generates a very large range of X-ray spectrum, thereby resulting in unnecessary exposure to radiation and degradation in image quality. Research has recently been conducted to develop good quality monochromatic x-rays by using carbon nanotubes, ultra-high energy laser, and the like.
In an aspect of detector, there has been research in improving sensitivity and a signal-to-noise ratio (SNR), and more recently, research has been actively conducted in developing a method for detecting single photon, leading to an expectation that detector performance may be enhanced to a higher level in the near future.
In an aspect of reconstruction, research has been conducted in developing a method for reducing noise of reconstructed CT images in a case where low X-ray dose or a small number of X-ray images are used. A method of repetitive reconstruction is mostly used to improve quality of CT images, but it presents the problem of taking a long time for reconstruction. However, a rapid development in GPU board as well as development in parallel processing allows repetitive reconstruction processing, even for a personal computer, within a few minutes.